The present invention relates to a method and an apparatus for correcting exposure patterns in a lithography process of semiconductor devices, as well as an exposure mask, a method of exposing, and a semiconductor device.
In a photo-lithography process for the manufacturing of semiconductor devices, an exposure mask provided with a mask pattern corresponding to a desired design pattern is prepared, and a light is exposed on a resist material through this mask to transfer the design pattern.
Recently, there was provided a lithography process executed around the limit of theoretical development corresponding to strictly specified design rules required as semiconductors are finely fragmented more and more. And, this often causes resolution to become insufficient, resulting in occurrence of a problem of difference between the mask pattern and the transferred resist pattern.
Such a phenomenon then causes deterioration of semiconductor device performance due to the transformation of the printed pattern, as well as a drop of production yield due to pattern bridging and wire breaking. To avoid such problems, therefore, the object mask pattern is optimized with the cut & try treatment to obtain a desired resist pattern.
There is also provided a treatment for optimizing a mask pattern such way; a plurality of qualification patterns are added to the object design pattern to correct the mask pattern, then a transferred pattern is determined by a printing test or simulation and obtain qualification patterns so as to find a transferred pattern closest to the desired design pattern.
However, it takes too much time and too many processes to determine an optimized mask pattern with the cut & try treatment. This method can therefore apply only to some patterns. The method cannot apply to such irregular patterns as an ASIC. In addition, the number of mask patterns that can be evaluated with the cut & try treatment is also limited, so that the optimized mask pattern might be overlooked.
To solve such problems and let each mask pattern be optimized by a computer automatically, a photo-proximity effect correction technology has been developed. In a mask pattern correction by this technology, the following processings are executed to correct the object mask pattern for the inputted design pattern.
(1) The visible outline of the inputted design pattern is divided into edges.
(2) An evaluation point is assigned to the center of each of the edges.
(3) The transferred image at the evaluation point is found, as well as the offset between this evaluation position and the position corresponding to the transferred image evaluation point is found. Otherwise, the offset of the energy intensity at the evaluation point from the desired value is found.
(4) Each edge is moved to a position so that the offset of the transferred image at the evaluation point becomes 0.
The above processings (3) and (4) are repeated by the computer to find the optimized mask pattern.
Correction of a mask pattern with such automatic photo-proximity effect correction technology will result in the following problems, however. As the offset (error) at the evaluation point is minimized, the error grows at portions to which no evaluation point is added, so that the offset of the mask pattern is also enlarged as a whole.
FIG. 1 shows an example of mask pattern correction in the related art. When an attempt is made to minimize the offset at the evaluation point shown with an x mark in the figure, part of the transferred image I formed using the corrected mask pattern is expanded significantly from the design pattern P depending on the shape of the design pattern P.
To avoid this problem, a method was considered for reducing the pitch between evaluation points by increasing the number of divisions for the design pattern P. This method, however, causes the mask pattern to be complicated after correction, so that the method makes it difficult to manufacture the object semiconductor device, as well as to increase the cost of the mask. In addition, since finely fragmented patterns are generated, it becomes impossible to make fault checks. And, it also becomes impossible to create quality-assured masks sometimes.
In addition, unnecessary steps are also developed in the mask pattern and such differences in level cause the contrast of the transferred image to be lowered. FIG. 2A shows a transferred image I of a design pattern P. FIG. 2B shows a transferred image I obtained by adding a corrected design pattern P' to the design pattern P. Although the same transferred image I is obtained for both design patterns P and P', the contrast of the transferred image I is lowered when the step is expanded by the corrected pattern P' added as shown in FIG. 2B. As a result, the exposure tolerance is lowered. This is another problem to occur in the related art.